Search took me to page about Malaysian flags but details appear relevent to British RASC.
A bit more info that may explain the quote :
" In 1945 it was established that the usual practise with flags of three colours was to have the lightest colour in the center and when the stripes were vertical, the darkest colour was to the mast. In 1946 it was discovered that white in heraldry was silver and that in heraldic law "metal cannot be placed on metal" i.e. the badge of gold on white which is argentinum or silver.
To overcome this problem the arrangement of the colour sequence since 1952 was gold to the mast, blue with badge defacement as it was known in heraldry and silver to the blast [i.e. fly]."
As the book is about the RASC during WW2 the inscription refers to the flag that was flown at that time rather than the later (post 1952) flag.
https://www.crwflags.com/fotw/flags/my%5Earmy.html
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